1. Field of the Invention
The invention relates to the uniform charging and discharging of electrochemical storage cells series-connected, or of groups of several parallel-connected storage cells series-connected to form a battery of the alkali metal and chalcogen type with at least one anode space for the accommodation of the anolyte and one cathode space for the accommodation of the catholyte, both spaces separated from each other by an alkali ion-conducting solid-electrolyte wall. More particularly, the present invention relates to a circuit with a protective element to effect uniform charging and discharging of electrochemical storage cells.
2. Description of the Prior Art
The subject of related application, U.S. Pat. No. 4,303,877 and corresponding German application No. P 28 19 584.8, is a circuit for the uniform charging and discharging of electrochemical storage cells which are either in series combined to form a battery, or in groups of several parallel-connected storage cells. The cells are constructed on the basis of employing alkali metal and chalcogen with at least one anode chamber for receiving the anolyte, and one cathode chamber for receiving the catholyte, which are separated from each other by an alkali ion-conducting wall of solid electrolyte. To each storage cell connected in series, or to each group of storage cells at least one protective element is connected in parallel which bridges (by-passes) their current flow when reaching a predetermined, maximal charging- or discharging level of the storage cell(s), and which is in connection with a control switch which is directly conductively connectible with the negative and positive electrodes of these storage cell(s) and the electrical connecting poles. The protective element is directly connected to the two electrical connecting poles, and the storage cell(s) is (are) at least connected to one of the two electrical connecting poles through a switch.
Such rechargeable electrochemical storage cells are very well suited for the construction of accumulators of high density energy and power. For example, the electrolyte of .beta.-aluminum oxide used at sodium/sulfur-storage cells allows only sodium ions to pass. This means, in contrast to the lead-accumulator, that practically no self-discharging can occur, and no side reactions take place, as, for example, a water decomposition at the lead/lead oxide system. Therefore, the current output, i.e. the Faraday-efficiency of a sodium/sulfur-storage cell is almost 100%.
In operation, these advantages are opposed by the disadvantage that such cells must not be overcharged or over-discharged (discharged in excess), as can be done with lead accumulators. The total capacity is determined by the cell with the lowest capacity, especially in a series circuit of cells. Especially important is the fact that when storage cells with a different charging state are placed in a series combination with others, for example, they can never be synchronized with the others in the line. With lead-accumulators it is possible to bring all cells to the same state by over-charging-hydrogen/oxygen formation (equalizing charge). To counteract this differently charged state of the storage cells of a battery, first several storage cells are connected in parallel, before several such groups of parallel-connected cells are connected in series. A further disadvantage of these electrochemical storage cells shows up when discharging a battery. For example, if one storage cell in a series circuit of many storage cells, or if the parallel-connected cells of a group in series with other groups, is already discharged, then the discharge current of the not yet discharged cells of the battery acts on the already discharged cells like an extraneously impressed current.
To assure the uniform charging and discharging of the storage cells of a battery, at least one protective element is connected in parallel with each storage cell in the series, or with each group of storage cells. This protective element by-passes the current flow of the storage cell(s) when it reaches a predetermined, maximal charge or discharge. In addition, this protective element is in connection with a control switch which is directly conductively connectible with the negative and positive electrodes of these storage cell(s) and the electrical connecting poles. The protective element is directly attached to both connecting poles, while the storage cell(s) is/are at least connected to one of the two connecting poles or terminals of the battery through a switch. The advantage of the circuit disclosed in the related application lies in the fact that with this circuit each storage cell of the battery can be charged to its maximal capacity. Furthermore, it also permits use of the storage cells in the battery which are in a different charging-state than the remainder of the storage cells, because with this circuit it is possible to achieve synchronization of these storage cells with the rest of the series string of individual or groups of storage cells in parallel. When storage cells are connected in parallel to form a group, due to equalizing current within the group, the same charge-condition exists in all storage cells.
The instant invention relates to an improvement and further development of the circuit described in the related application.
A certain disadvantage of the circuit disclosed in the related application can be seen in the fact that the adaptation to the threshold voltage and interior resistance of the storage cell(s) is often only possible by the series connection of several protective elements of the circuit. Furthermore, the elements used in the circuit are temperature sensitive, so that a direct installation of the circuit in the battery is not possible.